[Baalz]

The answer is because there are lots of common scenarios where the plate troops will last longer. Fighting hoplites mano-a-mano with no magic support may or may not be one of them.

Try walking into withering flaming arrow/crossbow fire - who lasts longer?

Try (as I mentioned) most melee after legions of steel has been cast? Measure not how many things they can kill, but how long they can hold off a superior force. These are linebackers, after all.

Test survivability against evocation spells - an extra bit of protection goes a long way even against AP damage because of the damage distribution curve I mention above.

Comparing the killing power of the Ulmish infantry is in my opinion missing the point. They're not there to kill stuff.